Background: Myalgic Encephalomyelitis/Chronic fatigue syndrome (ME/CFS) is an illness characterized by disabling fatigue. We examined the applicability of Waon therapy as a new method of fatigue treatment in patients with ME/CFS. Methods: Nine female ME/CFS patients (mean age, 38.4±11.2 years old; range, 21-60) who fulfilled the criteria of the Canadian clinical case definition of ME/CFS participated in this study. Patients received 30 sessions of modified Waon therapy, infrared-ray dry sauna maintained at an even temperature of 40°C or 45°C for 15 minutes twice a day for 3 weeks in a hospital, or once a day for five weeks at an outpatient clinic. Their functional health and well-being scores were determined using SF-36 and compared with those of six ME/CFS patients who did not undergo Waon therapy. Results: Seven of nine Waon therapy patients experienced a significant improvement in physical and mental condition, and the effect continued throughout the observation period. Waon therapy brought improvements in the scores of: Role physical (p<0.05); Bodily pain (p<0.05); General health perceptions (p<0.05); and Role emotional (p<0.05) of SF-36 in those who responded well (good responders) to the therapy. In two patients who responded poorly (poor responders) to Waon therapy, and in the non-Waon therapy patients, no significant improvement in the scores was observed. Conclusions: Waon therapy is effective for the treatment of ME/CFS.